Voltage grading rings on Lightning arrresters

[prc]

LAs are provided with metallic rings at the top terminal end (one small ring and below it another larger ring,reaching down to 30-40 % of LA height)These are not for mitigating the corona, but for distributing the power frequency electric field uniformly in the axial direction. In the days of gapped, silicon carbide arresters( before 1980s) voltage grading rings were required to prevent the gap spark over during wet conditions. With gapless MOV arresters of today, the dia of rings has come down (as axial height of LA came down. With out these grading rings, upper sections of Zn oxide blocks will be heated more due to non uniform, axial voltage sharing between sections of LA.

 

[electricpete]

good info. I've heard them [incorrectly] called corona rings and never thought about it much. But I see now from your discussion and from here that corona ring and grading rings are different things with different functions. And I guess there are multiple variations within the category of grading rings.

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(2B)+(2B)' ?

 

[prc]

Thank you pete, for that link. Wiki is marvelous-crisp, to the point. I was also having the same impression as you, but corrected by the writings of Jonathan J Woodworth -

http://www.arresterworks.com

is a great link for Application issues on Surge Arresters.